GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 19 and covered by Resource Consent 971311 (subject to condition 8b) from that time. In applying for this, the Applicant (OGNZL’s predecessor) proposed the following controls: In order for the collection ponds to be classified as silt ponds and allowed to direct discharge, the water quality within the individual ponds must have pH in the range of 6.5 to 9, and turbidity must be less than or equal to 110 NTU (equivalent to 100 g/m3 suspended solids). Flow from S3, S4 and S5 either individually (if discharged one at a time) or in combination will be managed such that it does not exceed 13% of the flow measured at the Ruddock’s flow gauge in the Ruahorehore Stream. Provided the above two conditions are met, the Silt Pond Discharge Consent 971311 will apply, and the conditions of that consent (which includes the receiving water standards) will be complied with. If the ponds do not meet these conditions then they will be classified as collection ponds and the conditions of the Collection Ponds consent 971312 will apply. A monitoring regime was also agreed at that time and since the direct discharge has been underway, modifications to this have been made progressively. This has included a change from reliance on sample collection during direct discharge events, to the use of automatic monitoring data (pH, turbidity) which was approved in June 2016. 2.5.2 Design Criteria Collection ponds are currently designed to contain rainfall volumes equivalent to a 10 year return period, 72 hour duration event. The large volume of storage provided for by these design criteria was intended to provide containment for all but high rainfall conditions. The criteria also recognised that the catchment response for the Ohinemuri River differs to the smaller collection pond catchments. The intent of the design was to time overflow from the collection ponds to coincide with peak flows in the receiving water, thus maximising the opportunity for dilution of the overflow water. 2.5.3 Waihi North Project The Waihi North Project will introduce TSF3 in the upper reaches of the Ruahorehore Stream catchment, and new collection ponds (S6 and S7) are proposed to collect runoff generated on associated embankments. There are also new collection ponds proposed to collect water generated from the NRS and WRS. Due to the nature of the surface water discharge environments for ponds associated with TSF3 and the WRS (into the Ruahorehore and Mataura River, respectively), it is considered that a design criteria to contain rainfall volumes equivalent to a 10 year return period, 24 hour duration event is appropriate. The reduced duration design event (compared with existing facilities at Waihi) reflects the shorter response time (in terms of peak flow) of these relativaly smaller receiving surface waters following high rainfall events (refer Section 5).
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